If I was defragging a PC I would not do ANYTHING else on it.
Also, a defrag can take over an hour (depending on size of hard disk and how disorganized it is).
Nobody has explained Defrag.
On a new PC or laptop, when Windows writes files to disk it puts them neatly one after the other.
Then as files get deleted, updated, added or moved then Windows put them all over the place on the disk.
Some large fles can even be split up into many files, scattered all over the disk.
So if you have a big movie file, you may see it as one file, but Windows may have it stored in bits all over the place.
The more FRAGMENTED your data is the more it slows Windows down (it spends longer hunting on the disk for the data)
If you run a defragmenter it tidies them all up by putting them all back neatly.
Because Windows is moving files all over the place during a defrag, this is why I would not use the PC.